Moveable micro-mirrors based on MEMS technology are used in numerous devices, for example, in miniaturized projection systems, and in visible or infrared light sensors, such as bar code readers. Of interest here are micro-mirrors attached to a frame by an axis and orientatable around this axis by electromagnetic means.
FIG. 1 schematically shows an MEMS structure, formed in a silicon wafer, including an electromagnetically actuated moveable micro-mirror. This structure comprises a reflective, moveable small plate 1, or micro-mirror, attached to a stationary frame 3. A gap 5 extends between moveable plate 1 and frame 3. Plate 1 is connected to frame 3 by two arms 7 and 9 aligned on both sides of the plate, along a same axis 11. Thus, plate 1 is rotatable around axis 11 formed by arms 7 and 9. The movement of plate 1 exerts torsion on arms 7 and 9.
A conductive path 13 follows the periphery of the front face of plate 1. Path 13 passes across arm 7 and ends in contacts 15 and 17 formed on frame 3. Contacts 15 and 17 are suited to be connected to a power source, which is not shown, in such a manner that a current flows in conductive path 13 in the direction represented by arrows 19 (in the case of direct current).
The assembly of frame 3 and plate 1 are subjected to a lateral magnetic field, represented by arrows 21, wherein the field lines are substantially perpendicular to axis 11 and substantially parallel to the plane of frame 3.
When a current flows through conductive path 13, opposite Laplace forces are exerted orthogonally to the plane of the frame, on the portions of path 13 parallel to axis 11, and in which opposite currents flow. These combined forces produce a rotation of plate 1 around its axis 11, of an angle determined in particular by the direction and intensity of the current. Therefore, it is possible to modulate the orientation and inclination of plate 1 by varying the sign and value of the voltage applied between contacts 15 and 17.
FIG. 2 shows a cross-sectional view that schematically represents an MEMS structure including an electromagnetically actuated micro-mirror of the type described in relation to FIG. 1. In this view, one can see that the silicon wafer is hollowed out below the location where moveable plate 1 is formed. A support 23 closes off this recess. Also generally provided above the micro-mirror is a cover, which is not shown and is preferably transparent to protect plate 1 from the intrusion of contaminants.
Two magnets 25 and 27 are placed symmetrically on both sides of axis 11. Magnets 25 and 27, having a lateral magnetic orientation create, in the area of mobile plate 1, a magnetic field whose field lines are orthogonal to axis 11 and parallel to the plane of frame 3.
A disadvantage of micro-mirror structures of the type described in relation to FIGS. 1 and 2 lies in the size associated with the placement of magnets 25 and 27.
In practice, moveable plate 1 may be a square measuring about 1 mm on a side, gap 5 may measure substantially 50 μm, and the frame may have a width of substantially 1 mm.
The surface area (as seen from above) of magnets 25 and 27 is added to the surface area of frame 3. And yet, magnets 25 and 27 are relatively distant from moveable plate 1. Therefore, to ensure a sufficient field in the area of moveable plate 1, they must have dimensions on the order of 2 mm wide, 2 mm thick, and 3 mm long. The total surface area of the structure is therefore at least doubled by the presence of magnets 25 and 27.
FIG. 3 schematically shows another MEMS structure including an electromagnetically actuated micro-mirror. For clarity's sake, only the differences in relation to FIG. 2 shall be detailed.
To limit the size, it is proposed to replace magnets 25 and 27 having a lateral magnetic orientation in FIG. 2 by a magnet 31 having a vertical magnetic orientation, placed under the assembly formed by moveable plate 1 and frame 3. Magnet 31 creates, in regard to the moveable plate, a magnetic field whose field lines are orthogonal to the plane of frame 3.
A conductive coil divided into two separate windings 33 and 35 of the opposite direction, arranged on the front face of plate 1, on the side of each of the edges of plate 1 that are parallel to axis of rotation 11, respectively, is provided. This coil is suitable to be connected to a power source.
When a current flows in the coil, opposing attractive and repelling forces are exerted between magnet 31 and each of the windings 33 and 35, resulting in a rotational movement of moveable plate 1 around its own axis 11.
A disadvantage of this type of micro-mirrors lies in the loss of usable surface on the front face of moveable plate 1, which is associated with the dimensions of windings 33 and 35. The mass of the moveable part will also be greater, which requires one to provide, for a given magnetic field, higher currents to result in its displacement. In addition, because of its increased mass, the moveable plate will be less resistant to impacts and accelerations.